1. Field of the Invention
This invention relates generally to the field of techniques for compressing still images containing color information. The invention is particularly suited for use in compression of images having objects of interest in one color and background objects either clear or of another contrasting color. An example of such images are images of cellular specimens, such as, for example, digital color images of a tissue sample obtained from a microscope equipped with a color camera, in which the sample is stained with one or more stains to highlight cellular structure, cellular objects, or other features such as positive objects, proteins, etc. However, the invention is applicable to compression of other types of images.
2. Description of Related Art
The basis for many data compression methods used today is the reduction, removal, or exploitation of statistical redundancy in the image. Image data is often highly spatially redundant. In particular, a given picture element or pixel is often partially correlated with its neighbor(s). For example, if an image has a significant amount of blank areas, any given pixel in the blank area is likely to have the same value or intensity as an adjacent pixel. Some popular image compression methods, whether lossy or loss less, work to exploit this redundancy to achieve compression.
Loss less dictionary based compression methods and substitution compression methods (e.g. LZW, WINZip) assign a “symbol” to each data value, or sequence of values. This symbol is transmitted or stored instead of the original data. Statistical redundancy in the original data results in this symbol being shorter, i.e. requires fewer bits, than the original data sequence, thereby resulting in compression. Statistical, or entropy coders (Shannon-Fano, Huffman, or Arithmetic) work similarly. These methods assign a relatively short binary sequence to the most frequently occurring data value or string, and longer sequences to those occurring less frequently which can result in compression when the original data contains redundancy. Predictive compression methods, e.g. Differential Pulse Code Modulation (DPCM) predict the value of a given sample based on the redundancy of previous data values and code the difference (only) thereby reducing redundancy. Transform compression methods Discrete Cosine Transform (DCT), Fourier, Wavelet, or other) achieve compression partly by reducing the coding precision of the transform coefficients but also by entropy coding. Lossy baseline JPEG compression works in this way, while loss less JPEG utilizes a mix of DPCM and entropy coding.
Color imagery is often compressed without regard to redundancy between red, green and blue color channels. For instance, in lossy baseline JPEG compression of 3-band color images, the initial red, green, and blue planes are transformed into a color space such as Hue, Saturation, and Intensity. The Hue and Saturation planes are down-sampled to reduce the total amount of data. These planes are subsequently up-sampled upon reconstruction making use of the reduced chrominance resolution capability of human color vision, without regard to spectral redundancy.
In the biology fields, including cytology, histology, and pathology, digital images of tissue and cellular objects are typically obtained from a microscope equipped with a color camera which records red, green, and blue planes for these images. Frequently, the objects in the specimen can fall into two general types: normal cells and abnormal cells. The images typically include clear areas of background, representing inter-cellular spaces. It is also common practice to apply one or more stains to the specimen on the slide so that the objects of interest have a contrasting color from background objects or objects of less interest so that they are more readily identified and observed. For example, normal cells are often stained (or, counterstained as is usually said) with a stain such Hematoxylin and appear light blue, while abnormal cells (i.e. positive cells) are stained with a different stain, such as 3-amino 9-ethylcarbazol (AEC) so that the abnormal cells have a different color, e.g. reddish brown. Other color combinations are possible.
The present invention provides methods and apparatus for compression of color images with little or no loss of useful image information. Techniques for compression of digital images without significant loss of image information, such as provided with this invention, are useful to the art because they reduce the bandwidth requirements for transmission of such images over computer networks, thereby allowing such images, or groups of images, to be sent quickly from one location to another.